feine intake and breast cancer in heavier women (8).
Some studies of daily or intermittent caffeine intake in
male animals report elevations of testosterone levels
(9, 10). Associations in humans may be further confounded because caffeine consumption is negatively
associated with age and positively associated with
cigarette smoking and alcohol intake (7, 11).
The community-based Rancho Bernardo study of
older adults in southern California provided an opportunity to study whether caffeine intake in postmenopausal women is independently associated with endogenous sex hormone levels.

It has been hypothesized that intake of caffeinecontaining beverages may play a role in the development of hormone-dependent conditions such as osteoporosis (1), breast cancer (2, 3), fibrocystic breast
condition (4), and endometriosis (5). One large crosssectional study of perimenopausal women reported a
negative correlation between caffeine and the percentage of bioavailable estradiol (but not total estradiol or
estrone levels) (6). In another study, however, ageadjusted caffeine consumption was not associated with
estradiol or estrone levels in pre- or postmenopausal
women (7). Some studies suggest differential effects
of caffeine depending on body weight with an inverse
association with caffeine intake and breast cancer in
lean women and a positive association between caf-

MATERIALS AND METHODS

Participants were part of an ongoing communitybased study designed to examine lifestyle and healthy
aging among middle- to upper-middle-class older
adults in Rancho Bernardo, California. From 1984
through 1987, 1,385 women aged 24-90 years completed a standard questionnaire on diet, smoking,
physical activity, and alcohol consumption by telephone or clinic visit. Self-reported physical activity

Received for publication September 15, 1995, and accepted for
publication April 24, 1996.
1
General Preventive Medicine Residency Program, University of
California, San Diego/San Diego State University, San Diego, CA.
2
Department of Family and Preventive Medicine, University of
California, San Diego, San Diego, CA.
Reprint requests to Dr. Elizabeth Barrett-Connor, Department of
Family and Preventive Medicine, University of California, San Diego,
9500 Gilman Drive, La Jolla, CA 92093-0607.

642

Downloaded from aje.oxfordjournals.org by guest on November 26, 2010

Caffeine intake has been associated with risk of osteoporosis, breast cancer, endometriosis, and fibrocystic
breast disease and has been hypothesized to exert its effects through alteration of endogenous hormone
levels. This study examined the cross-sectional association of caffeine intake with endogenous androgens,
estrogens, and sex hormone-binding globulin in 728 white postmenopausal women aged 42-90 years in the
Rancho Bernardo community-based study in 1984-1987. Caffeine intake was inversely associated with age
and waist/hip ratio and positively associated with alcohol consumption. Significant inverse associations were
noted between caffeine intake and bioavailable testosterone, which persisted after adjustment for age,
waist/hip ratio, body mass index, alcohol intake, cigarette smoking, and physical activity (r = -0.10,p = 0.02).
At high doses (equivalent to more than 2 cups of coffee or four cans of caffeinated soda daily), caffeine intake
was positively associated with plasma estrone before and after adjustment for confounders (r = 0.26, p =
0.05). Sex hormone-binding globulin levels were positively associated with increasing caffeine intake (adjusted
r = 0.09, p = 0.03). The positive association of caffeine with estrone and its inverse association with
bioavailable testosterone suggest that caffeine's reported association with several chronic conditions may be
mediated by an effect on endogenous sex steroids. Am J Epidemiol 1996; 144:642-4.

was ascertained by asking participants whether they
engaged in strenuous activity three or more times a
week (yes/no). Caffeine intake was ascertained using
the Harvard-Willett semiquantitative food frequency
questionnaire (12). Height, weight, and waist and hip
circumference were measured at a clinic visit. Sex
hormone levels were measured in all 728 postmenopausal women not taking exogenous hormones. Venipuncture was performed in fasting subjects between 7
and 11 a.m.; plasma was stored at -70°C until first
thawed for sex hormone analyses in 1990-1991. Previous work has demonstrated no deterioration of hormone levels over 15 years when samples are frozen
and stored in tightly sealed containers (S. S. C. Yen,
unpublished data). Estrone, estradiol, and testosterone
were measured by radioimmunoassay (13). Bioavailable testosterone and bioavailable estradiol (non-sex
hormone-binding globulin bound) were measured using the method of Tremblay and Dube (14). Sex hormone-binding globulin, which binds testosterone and
estradiol, was determined by the method of Rosner
(15). Women with estrone levels greater than 76 pg/ml
(n = 5) or estradiol levels greater than 35 pg/ml (n =
10) were excluded because these hormone levels are
consistent with premenopausal status or undisclosed
estrogen use.
For estradiol and bioavailable estradiol, 14 percent
(101 of 728 women) had values below the sensitivity
of the assay. Four women (0.5 percent) had values
below the sensitivity of the assay for testosterone and
its bioavailable fraction; 0.4 percent of women had
values below assay sensitivity for estrone. Undetectable levels were converted to levels just below the
sensitivity of the assay for analysis. Intraassay coefficients of variation and sensitivity were as follows:
testosterone (4.00 percent, 20 pg/ml), estrone (6.00
percent, 3 pg/ml), estradiol (5.87 percent, 3 pg/ml),

A literature survey revealed no previous work examining the effect of caffeine intake on testosterone
levels in postmenopausal women, hi this analysis,
bioavailable testosterone was negatively and independently associated with caffeine intake, while estrone
was positively associated with high levels of caffeine
consumption. Sex hormone-binding globulin was positively associated with caffeine intake, replicating
findings of London et al. (6). Sex hormone-binding
globulin is the major carrier of steroid hormones in the
circulation and an important moderator of bioavailable
hormone levels. The higher level of binding globulin
probably explains the observed reduction in bioavailable testosterone levels. A similar reduction in bioavailable estradiol may not be apparent because these
levels are already very low in postmenopausal women.
Because sex hormone-binding globulin is produced in
the liver, the results suggest a caffeine effect on hepatic metabolism. These findings are compatible with
the effect of caffeine on medications metabolized by
the liver (16).
These intriguing caffeine-hormone associations may
explain some of the previously reported associations
between hormone-related conditions and the intake
of caffeinated beverages. If caffeine increases sex
hormone-binding globulin levels, this may result in
decreased levels of bioavailable estradiol and testosterone, providing one possible mechanism for both a
diminished breast cancer risk (8) and an increased risk
of osteoporosis (17) reported among caffeine users.
The finding of higher levels of estrone in those with
high caffeine intake may partially account for the
association between caffeine consumption and endometriosis, even though the present study examined
postmenopausal women (5). The absent expected associations with cigarette smoking and body mass index probably reflect the small number of smokers (12
percent) and overweight women (body mass index
&gt;27 = 22 percent) in this older population.

This research was supported by National Institute of
Diabetes and Digestive and Kidney Diseases grant
DK31801 and National Institute on Aging grant AGO7181.
Dr. Ferrini was supported by the American Cancer Society
Physician Training Award in Preventive Medicine.